Apparatus for optically inspecting the condition of a surface having known variations in the condition

ABSTRACT

A laser beam is used to selectively scan a surface under test, the characteristics of the surface determining the manner in which the scanning beam is reflected. A selected portion of the reflected beam is thereafter analyzed to indicate the condition of the surface.

[451 Sept. 25, 1973 APPARATUS FOR OPTICALLY INSPECTING THE CONDITION OFA SURFACE HAVING [5 6] References Cited UNITED STATES PATENTS KNOWNVARIATIONS IN THE CONDITION MN I e m $1 l om CT 7200 6777 9999 HHHH70026 678 66700 ,93 065 584 3333 K x r h 0 w Y e l w a m aN n e n m 00 wmm r k m. w m .m m n .m S ID. 3 mm m r 0C Bk h me TN IT w w n m g e w hA H U 7 7 Primary Examiner-Ronald L. Wibert Assistant Examiner Paul K.Godwin Attorney-James B..Raden 22 Filed: Jan. 17, 1972 Appl. No.:218,099

[57] ABSTRACT A laser beam is used to selectively scan a surface underRelated U.S. Application Data Continuation of Serl No. 869,612, Oct. 27,abandoned test, the characteristics of the surface determining themanner in which the scanning beam is reflected. A selected portion ofthe reflected beam is thereafter analyzed to indicate the condition ofthe surface.

16 Claims, 5 Drawing Figures PATENIEDSEPZSBH 3.761.188

SHEET 2 BF 2 C LT, I LV Path of Scan-* Fig. 5.

INVENTOR.

Thomas 0. 1L:

APPARATUS FOR OPTICALLY INSPECTING TlliilE CONDITION OF A SURFACE HAVINGKNOWN VARIATIONS IN THE CONDITION quently result in impairing theiroperation. However,

due to their configurations, such subassemblies often do not readilylend themselves to microscopic inspection using standard techniquessince the equipment normally required therefor usually cannot gainaccess to the cavity under inspection. Furthermore, in light of the highvolume of parts produced, it is important that the inspection techniquesimplemented be quick, efficient and not rely on the subjectivedetermination of an individual inspector. Ideally, it is desirable toutilize techniques which lend themselves to automation.

Accordingly, it is an object of the present invention to provide a meansfor optically inspecting the condition of a surface.

A further object of the present invention is to provide a means foroptically inspecting the interior surface of a cylindrical bore, or likeconfiguration, for purposes of detecting the existence of flaws thereon.

Apparatus for optically inspecting the condition of a surface, inaccordance with the present invention, comprises: means for providing acollimated laser beam; means for scanning the surface under test withsaid laser beam, said beam being reflected by said scanned surface inaccordance with the condition thereof; means for collecting that portionof said reflected beam which is confined to a predetermined region, saidcollected portion being indicative of the condition of the surface underinspection; and means for deriving an output signal from said collectedbeam.

In addition, apparatus in accordance with the present inventionpreferably includes means preprogrammed to disregard said collected beamwhere its characteristics are influenced by the existence of knownsurface conditions.

The present invention, along with additional objects and advantagesthereof, will be more fully understood upon reading the specificationwhich follows in light of the accompanying drawing wherein:

FIG. 1 illustrates an optical scanning device in accordance with oneembodiment of the present invention;

FIG. 2 is a sectional view taken along section 22 of FIG. I,illustrative of one embodiment of the aforementioned preprogrammed meansused to disregard the optical signal where its characteristics areinfluenced by the existence of known surface conditions;

FIG. 3 illustrates one manner of integrating the preprogrammed meansshown in FIG. 2 with the scanning apparatus of FIG. 1;

FIG. 4 illustrates the manner in which the scanning beam isreflected asa function of the characteristics of the surface under inspection; and

FIG. 5 represents wave forms as they appear at the points indicated inFIG. 3.

Turning now to a description of FIG. 1, a source of laser energy 10 isprovided, the output of which is directed through a subsystem 12 toprovide a beam of laser light 13 exhibiting predeterminedcharacteristics. The subsystem 12, for example, may be a beam expanderor a reducing collimator. Should it be desirable to provide a laser beamhaving a uniformly distributed energy density across its diameter, aspacial filter such as described in US. Pat. No. 3,465,347 may beincorporated. The use ofa subsystem has been found to be optional anddependent solely on the degree of resolution desired. In practice, ahelium neon laser having a 1 milliwatt output at 6321 angstroms has beensatisfactorily used in conjunction with a reducing collimator.

A drilled diagonal mirror 14 is disposed nonorthogonally in the path oflaser beam 13, the diagonal mirror 14 being placed to permit thecollimated beam 13 to pass directly through the hole therein.

A probe assembly 20 comprising a main tubular portion 22, a steppedpully block 24, and a pair of spindle bearings 26, is disposed coaxiallywith the collimated beam 13. The tubular portion 22 of the probeassembly 20 is adapted for insertion within the unit under test 30, andis formed with a hollow interior cavity such that the laser beam 13 canbe projected along the longitudinal axis thereof. In the embodimentrepresented by FIG. 1, the unit under test 30 might be, for example, amaster cylinder having an open-ended cylindrical bore 32.

The front end of the probe assembly 20 supports an angled mirror 28whose reflective surface faces the hollow interior of the tubular probe.Alternatively, an internally reflecting prism may be used in place ofthe angled mirror 28, though the use of a prism may give rise toproblems due to surface back-scatter. A drilled lens 29 is disposed inthe path of the incoming laser beam 13 such that the beam initiallypasses through the hole in the lens 29 on course to the reflectivesurface of the mirror 28. Means (not shown) are provided for rotatablydriving the probe assembly 20 at a desired speed. For example, a drivebelt may be connected to one sheave of the pulley block 24.Alternatively, means other than a rotatably driven pulley block may beused to drive the probe assembly. For example, a directly driven airturbine may be used where high speed operation is desired.

As the probe assembly 20 is rotated within the unit under test 30, thelaser beam 13 passing through the drilled lens 29 is reflected off thesurface of the angled mirror 28 continuously scanning the interiorsurface of the cavity under test.

In practice, the bore surface behaves as a screen, serving to diffuseand scatter the light impinging thereon. The intensity of the reflectedlight is expressed by Lamberts +equation:

I 1,, Cos "6 where:

I is the maximum intensity of the reflected light;

I, is the intensity of the reflected normal component;

0 is the angle between any component and the normal; and

X is indicative of the optical characteristics of the reflectingsurface.

As the laser beam is scanned about the surface under test, it will bereflected in accordance with the optical characteristics thereof whichthemselves are representative of the physical condition of the surface.FIG. 4 illustrates the shape of the reflected energy distributions forsurfaces of like absorption characteristics exhibiting X values of 2 and3 (i.e., curves A & B). It will be seen that for values of 6 O, (i.e.,Cos fl =1 1,, is a function of the absorption characteristics of thesurface. Furthermore, it will be seen that generally the distribution ofthe reflected energy is a function of both the absorption and Xcharacteristics of the surface. Hence. with regard to the lens systemdepicted in FIG. 4, the illumination of the lens 29 can bemathematically expressed as Illum.=f0 I COS xg g ir and that for twosurfaces having the same absorption characteristics and differentdegrees of optical roughness, (i.e., different X characteristics) theone with the smoother surface (i.e., the B curve representing the higherX characteristic) reflects more light to the lens 29. Moreover, asurface flaw tends to exhibit both a lower X value and a lower degree ofabsorption. Accordingly, the A and C distributions shown in FIG. 4 aremost representative of reflections from a surface with (A) and without(C) a flaw.

In the configuration of FIG. 1, the reflected beam is directed onto lens29 by the angled mirror 28. The lens 29 serves to collect that portionof the reflected beam which impinges thereon, the collected portionbeing indicative of the condition of the surface under inspection.Thereafter, the collected portion is projected down the length of theprobe 22 so that it is reflected via the non-orthogonally disposedmirror 14 onto a photodetector 42 via lens 40. The photodetector 42serves to transform the light focused thereon into an output signalwhich is then fed to an analysis circuit (not shown).

The characteristics of the output signal may be described as an ACsignal which varies as a function of the quality of the surface underinspection, at a DC level. The signal may be fed to an oscilloscope andtransformed into a visual analogue on the display tube thereof. A sampleoutput, as might be seen at the output of photodetector 42, is depictedby curve a of FIG. 5, where the peak-to-peak amplitude of the AC signalis indicative of the quality of the surface under inspection. In theevent of a flaw 62, the amount of light upon photodetector 42 decreases,causing a dip in the output signal.

In many instances the surface of the unit under test may be formed in amanner which would indicate that a flaw exists when in reality theindicated flaw is intended. For example, the surface may be formed withsmall screw holes or with protruding portions such as are occasioned bychaplet wires. In such event it becomes difficult for the inspectingoperator or processing electronics, as the case may be, to distinguishbetween intended and non-intended flaws. To obviate the probability oferror, a gating circuit 50, as illustrated in cross section in FIG. 2,may be incorporated into the inspection apparatus.

Basically, the gating circuit 50 may comprise a focused light source 51which is reflected from a preprogrammed reference surface onto aphotodetector 56. The photodetector 56 is thereafter coupled to theanalysis circuit mentioned supra, via a gate 44 as shown in FIG. 3, suchthat in the absence of an output from the photodetector 56 the analysiscircuit 46 will not operate.

In FIG. 2, the light source 51 is focused via a lens 52 onto the sleeveof the tubular portion 22 shown in cross section of the probe assembly20. The sleeve is strategically marked along its length (i.e.,programmed) with a light absorbing material at points which correspondto the presence of known flaws. In the absence of a' mark on therotating probe sleeve 22, the light 53 from the light source 50 will befocused via lens 52 on the rotating probe sleeve 22 and reflectedtherefrom via lens system 54 onto photodetector 5 6; the output signalfrom photodetector 56 serving as a go" signal for the analysis circuit46 shown in FIG. 3. Where the probe 22 is marked with a light absorbingmaterial, indicative of the existence of a known flaw, the light outputof light source 51 will be absorbed thereby, and no light reflected tophotodetector 56. Consequently the analysis circuit 46 will be preventedfrom operating or gated." This programmedpredictability of intendedsurface irregularities permits the generated output signal to beelectronically processed, disregarding the gated spots, and obviates theneed for human intervention, thereby making the system particularlyadaptable for automated production line techniques. By properlypreprogramming the sleeve, the gating circuit can be adapted todisregard intended flaws regardless of their configuration.

Curve b of FIG. 5 depicts those flaws, within the output signalgenerated from photodetector 42 (i.e., curve a) which are intended.Curve 0 depicts the output from photodetector 56 which is thereafter fedto the gate 44 of FIG. 3. Curve d illustrates the final output signaltransmitted to the analysis circuit 46 with the dips 62 indicated due tointended flaws gated out.

In operation the unit under test 30 may be maintained stationary and thetubular ponion 20 of the probe 22 inserted therein and withdrawntherefrom .via means 34 mechanically coupled thereto. Alternatively, insome instances it may be desirable to maintain the rotating probe insitu" and advance the unit under test 30 about the tubular portion 22thereof.

I claim:

1. Apparatus for inspecting the condition of a surface having knownirregularities thereon, comprising:

scanning. means for optically scanning a surface under test inaccordance with a given program, said scanning means being responsive tovariations in the condition of said surface from a given condition, saidscanning means generating condition variation signals in response toboth known and unknown irregularities,

means coupled to said scanning means and preprogrammed to follow saidgiven program and to disregard said known irregularities whereby onlysignals for unknown irregularities are provided as output signals,

the surface to be inspected comprising a cylindrical bore, said scanningmeans including a probe rotatably and axially moveable in a given orderwith respect to said bore,

said preprogrammed means including means synchronized with said scanningmeans and means simulating said known condition for generating in timecoincidence with said known irregularities error cancelling signals forcancelling erroneous condition variation signals, and

said error cancelling signals causing said scanning means to disregardsaid known irregularities.

2. The apparatus of claim 1 wherein said preprogrammed means includesmeans secured to said probe external to said bore during the scanning ofsaid bore, said secured means having a given position external said borewhen said scanning means responds to said known irregularities, and

said preprogrammed means further includes means located at said givenposition responsive to said secured means for generating said errorcancelling signals. 3. The apparatus of claim ll wherein said securedmeans includes light absorbing material located on said probe atdiscrete locations, each discrete location corresponding to a separatedifferent one of said irregular ities, said probe being lightreflective, and

said means located at said given position includes light generatingmeans for causing a light beam to impinge upon said probe at said givenposition, and means responsive to the light reflected from said probe atsaid given position to generate an error cancelling signal whenever saidlight absorbing material is located in said given position.

4. Apparatus for inspecting the condition of a surface having knownirregularities thereon, comprising:

means for scanning the surface under test with a beam of light in agiven sequence, said scanning means being responsive to variations inthe condition of said surface from a given condition, said scanningmeans generating erroneous condition variation signals in response tosaid known irregularities,

said surface to be' inspected including a cylindrical bore,

preprogrammed means, including means simulating a known condition,coupled to said scanning means programmed to generate error cancellingsignals for causing said scanning means to disregard said erroneouscondition variation signals, an error cancelling signal being generatedfor each of said irregularities whereby said erroneous signals arecancelled.

5. The apparatus of claim 4 wherein said scanning means includes meansfor causing said beam of light to continuously scan said surface, saidknown irregularities occurring in a given order during said sequence.

6. The apparatus of claim 5 wherein the surface to be inspected is acylindrical bore, said scanning means including an elongated cylindricalprobe rotatably-and axially moveable with respect to said bore, and

said preprogramrned means includes a pattern of substantially identicalirregularities secured externally to said probe, arranged insubstantially the same pattern as the known irregularities on saidsurface, each of said identical irregularities corresponding to adifferent one of said known irregularities and being located in a givenposition external the bore when said light beam impinges upon thecorresponding known irregularity.

7. An apparatus for inspecting the condition of an interior surface of ahollow article, said surface having known irregularities thereon,comprising:

means for directing a narrow beam of radiant energy in a given sequenceover said surface and sensitive to the response of said surface to saidbeam of energy,'said means providing a first signal when said surfacevaries from a given condition, said first signal including an erroneouscondition variation signal when the surface variations include saidknown irregularities, said known irregularities occurring in a givenorder during said sequence,

means coupled to said beam directing means preprogrammed to provide asecond signal whenever said beam of energy impinges upon any of saidknown irregularities, said preprogrammed means including light sensitivemeans secured to said beam directing means in saidgiven order, lightgenerating means secured to radiate a beam of light on said lightsensitive means only when said beam of energy impinges upon said knownirregularities, transducer means responsive to light impinging upon saidlight sensitive means for generating said second signal whenever saidbeam of energy impinges upon said known irregularities, and meansresponsive to said first and second signals for providing a surfacevariation signal only when said first signal is generated in the absenceof said second signal whereby said erroneous condition variation signalis disregarded.

8. Apparatus for inspecting the condition of a surface having knownirregularities thereon, comprising:

means for optically scanning a surface under test in accordance with agiven program, said scanning means being responsive to variations in thecondition of said surface from a given condition, said scanning meansgenerating condition variation signals in response to irregularitiesincluding erroneous condition variation signals in response to saidknown irregularities,

means including a separate light source and light sensing means foroptically scanning a record of known irregularities and generatingblanking signals synchronized with said erroneous condition variationsignals, and

gating means coupled to receive said erroneous signals and said blankingsignals and to provide output signals incorporating said conditionvariation signals free of erroneous condition variation signals.

9. Apparatus for inspecting the condition of a surface having knownirregularities thereon, comprising:

means for optically scanning the surface under test in accordance with agiven program, said scanning means being responsive to variations in thecondition of said surface from a given condition, said scanning meansgenerating erroneous condition variation signals in response to saidknown irregularities,

means coupled to said scanning means and preprogrammed to follow saidgiven program for causing said scanning means to disregard said knownirregularities whereby only said erroneous signals are eliminated,

said surface to be inspected consisting of a cylindrical bore, saidscanning means including a probe rotatably and axially moveable withrespect to said bore, and

said preprogrammed means including means simulating said known conditionfor generating in time coincidence with said known irregularities errorcancelling signals for cancelling said erroneous condition variationsignals,

said error cancelling signals causing said scanning means to disregardsaid known irregularities.

10. The apparatus of claim 9 wherein said scanning means includes meansfor longitudinally and transversely scanning said surface in a givenorder, and

said preprogrammed means includes means synchronized with said scanningmeans.

11. The apparatus of claim 9 wherein said preprogrammed means includesmeans secured to said probe external said bore during the scanning ofsaid bore, said secured means having a given position external said borewhen said scanning means responds to said known irregularities, and

said preprogrammed means further includes means located at said givenposition responsive to said secured means for generating said errorcancelling signals.

12. The apparatus of claim 9 wherein said secured means includes lightabsorbing material located on said probe at discrete locations, eachdiscrete location corresponding to a separate different one of saidirregular ities, said probe being light reflective, and

said means located at said given position includes light generatingmeans for causing a light beam to impinge upon said probe at said givenposition, and means responsive to the light reflected from said probe atsaid given position to generate an error cancelling signal whenever saidlight absorbing material is located in said given position.

13. Apparatus for inspecting the condition of a surface having knownirregularities thereon, comprising:

means for scanning the surface under test with a beam of light in agiven sequence, said scanning means being responsive to variations inthe condition of said surface from a given condition, said scanningmeans generating erroneous condition variation signals in response tosaid known irregularities,

preprogrammed means coupled to said scanning means programmed togenerate error cancelling signals for causing said scanning means todisregard said erroneous condition variation signals, an errorcancelling signal being generated for each of selected ones of saidirregularities whereby only said selected ones of said erroneous signalsare cancelled,

said scanning means including means for causing said beam of light tocontinuously scan said surface, said known irregularities occurring in agiven order during said sequence,

the surface to be inspected including a cylindrical bore, said scanningmeans including an elongated cylindrical probe rotatably and axiallymoveable with respect to said bore, and

said preprogrammed means including a pattern of substantially identicalirregularities secured externally to said probe, arranged insubstantially the same pattern as the known irregularities on saidsurface, each of said identical irregularities corresponding to adifferent one of said known irregularities and being located in a givenposition external the bore when said light beam impinges upon thecorresponding known irregularity.

14. Apparatus for inspecting the condition of a surface having knownirregularities thereon, comprising:

scanning means for optically scanning the surface under test inaccordance with a given program, said scanning means being responsive tovariations in the condition of said surface from a given condition, saidscanning means generating condition variation signals in response toboth known and unknown irregularities,

means coupled to said scanning means and preprogrammed to follow saidgiven program and to disregard said known irregularities whereby onlysignals for unknown irregularities are provided as output signals,

said surface to be inspected including a cylindrical bore, said scanningmeans including a probe rotatably and axially moveable with respect tosaid bore,

said preprogrammed means including means simulating said known conditionfor generating in time coincidence with said known irregularities errorcancelling signals for cancelling erroneous condition variation signals,

said error cancelling signals causing said scanning means to disregardsaid known irregularities,

said preprogrammed means including means secured to said probe externalto said bore during the scanning of said bore, said secured means havinga given position external said bore when said scanning means responds tosaid known irregularities, and

said preprogrammed means further including means located at said givenposition responsive to said secured means for generating said errorcancelling signals.

15. The apparatus of claim 14 wherein said secured means includes lightabsorbing material located on said probe at discrete locations, eachdiscrete location corresponding to a different one of saidirregularities, said probe being light reflective, and

said means located at said given position includes light generatingmeans for causing a light beam to impinge upon said probe at said givenposition, and means responsive to the light reflected from said probe atsaid given position to generate an error cancelling signal whenever saidlight absorbing material is located in said given position.

16. Apparatus for inspecting the condition of a surface having knownirregularities thereon, comprising:

means for scanning the surface under test with a beam of light in agiven sequence, said scanning means being responsive to variations inthe condition of said surface from a given condition, said scanningmeans generating erroneous condition variation signals in response tosaid known irregularities,

preprogrammed means coupled to said scanning means programmed togenerate error cancelling signals for causing said scanning means todisregard said erroneous condition variation signals, an errorcancelling signal being generated for each of said irregularitieswhereby said erroneous signals are cancelled,

said scanning means including means for causing said beam of light tocontinuously scan said surface, said known irregularities occurring in agiven order during said sequence,

said surface to be inspected being formed as a cylindrical bore, saidscanning means including an elongated cylindrical probe rotatably andaxially moveable with respect to said bore, and

said preprogrammed means including a pattern of substantially identicalirregularities secured externally to said probe, arranged insubstantially the same pattern as the known irregularities on saidsurface, each of said identical irregularities corresponding to adifferent one of said known irregularities and being located in a givenposition external the bore when said light beam impinges upon thecorresponding known irregularity.

1. Apparatus for inspecting the condition of a surface having knownirregularities thereon, comprising: scanning means for opticallyscanning a surface under test in accordance with a given program, saidscanning means being responsive to variations in the condition of saidsurface from a given condition, said scanning means generating conditionvariation signals in response to both known and unknown irregularities,means coupled to said scanning means and preprogrammed to follow saidgiven program and to disregard said known irregularities whereby onlysignals for unknown irregularities are provided as output signals, thesurface to be inspected comprising a cylindrical bore, said scanningmeans including a probe rotatably and axially moveable in a given orderwith respect to said bore, said preprogrammed means including meanssynchronized with said scanning means and means simulating said knowncondition for generating in time coincidence with said knownirregularities error cancelling signals for cancelling erroneouscondition variation signals, and said error cancelling signals causingsaid scanning means to disregard said known irregularities.
 2. Theapparatus of claim 1 wherein said preprogrammed means includes meanssecured to said probe external to said bore during the scanning of saidbore, said secured means having a given position external said bore whensaid scanning means responds to said known irregularities, and saidpreprogrammed means further includes means located at said givenposition responsive to said secured means for generating said errorcancelling signals.
 3. The apparatus of claim 1 wherein said securedmeans includes light absorbing material located on said probe atdiscrete locations, each discrete location corresponding to a separatedifferent one of said irregularities, said probe being light reflective,and said means located at said given position includes light generatingmeans for causing a light beam to impinge upon said probe at said givenposition, and means responsive to the light reflected from said probe atsaid given position to generate an error cancelling signal whenever saidlight absorbing material is located in said given position.
 4. Apparatusfor inspecting the condition of a surface having known irregularitiesthereon, comprising: means for scanning the surface under test with abeam of light in a given sequence, said scanning means being responsiveto variations in the condition of said surface from a given condition,said scanning means generating erroneous condition variation signals inresponse to said known irregularities, said surface to be inspectedincluding a cylindrical bore, preprogrammed means, including meanssimulating a known condition, coupled to said scanning means programmedto generate error cancelling signals for causing said scanning means todisregard said erroneous condition variation signals, an errorcancelling signal being generated for each of said irregularitieswhereby said erroneous signals are cancelled.
 5. The apparatus of claim4 wherein said scanning means includes means for causing said beam oflight to continuously scan said surface, said known irregularitiesoccurring in a given order during said sequence.
 6. The apparatus ofclaim 5 wherein the surface to be inspected is a cylindrical bore, saidscanning means including an elongated cylindrical probe rotatably andaxially moveable with respect to said bore, and said preprogrammed meansincludes a pattern of substantially identical irregularities securedexternally to said probe, arranged in substantially the same pattern asthe known irregularities on said surface, each of said identicalirregularities corresponding to a different one of said knownirregularities and being located in a given position external the borewhen said light beam impinges upon the corresponding known irregularity.7. An apparatus for inspecting the condition of an interior surface of ahollow article, said surface having known irregularities thereon,comprising: means for directing a narrow beam of radiant energy in agiven sequence over said surface and sensitive to the response of saidsurface to said beam of energy, said means providing a first signal whensaid surface varies from a given condition, said first signal includingan erroneous condition variation signal when the surface variationsinclude said known irregularities, said known irregularities occurringin a given order during said sequence, means coupled to said beamdirecting means preprogrammed to provide a second signal whenever saidbeam of energy impinges upon any of said known irregularities, saidpreprogrammed means including light sensitive means secured to said beamdirecting means in said given order, light generating means secured toradiate a beam of light on said light sensitive means only when saidbeam of energy impinges upon said known irregularities, transducer meansresponsive to light impinging upon said light sensitive means forgenerating said second signal whenever said beam of energy impinges uponsaid known irregularities, and means responsive to said first and secondsignals for providing a surface variation signal only when said firstsignal is generated in the absence of said second signal whereby saiderroneous condition variation signal is disregarded.
 8. Apparatus forinspecting the condition of a surface having known irregularitiesthereon, comprising: means for optically scanning a surface under testin accordance with a given program, said scanning means being responsiveto variations in the condition of said surface from a given condition,said scanning means generating condition variation signals in responseto irregularities including erroneous condition variation signals inresponse to said known irregularities, means including a separate lightsource and light sensing means for optically scanning a record of knownirregularities and generating blanking signals synchronized with saiderroneous condition variation signals, and gating means coupled toreceive said erroneous signals and said blanking signals and to provideoutput signals incorporating said condition variation signals free oferroneous condition variation signals.
 9. Apparatus for inspecting thecondition of a surface having known irregularities thereon, comprising:means for optically scanning the surface under test in accordance with agiven program, said scanning means being responsive to variations in thecondition of said surface from a given condition, said scanning meansgenerating erroneous condition variation signals in response to saidknown irregularities, means coupled to said scanning means andpreprogrammed to follow said given program for causing said scanningmeans to disregard said known irregularities wherEby only said erroneoussignals are eliminated, said surface to be inspected consisting of acylindrical bore, said scanning means including a probe rotatably andaxially moveable with respect to said bore, and said preprogrammed meansincluding means simulating said known condition for generating in timecoincidence with said known irregularities error cancelling signals forcancelling said erroneous condition variation signals, said errorcancelling signals causing said scanning means to disregard said knownirregularities.
 10. The apparatus of claim 9 wherein said scanning meansincludes means for longitudinally and transversely scanning said surfacein a given order, and said preprogrammed means includes meanssynchronized with said scanning means.
 11. The apparatus of claim 9wherein said preprogrammed means includes means secured to said probeexternal said bore during the scanning of said bore, said secured meanshaving a given position external said bore when said scanning meansresponds to said known irregularities, and said preprogrammed meansfurther includes means located at said given position responsive to saidsecured means for generating said error cancelling signals.
 12. Theapparatus of claim 9 wherein said secured means includes light absorbingmaterial located on said probe at discrete locations, each discretelocation corresponding to a separate different one of saidirregularities, said probe being light reflective, and said meanslocated at said given position includes light generating means forcausing a light beam to impinge upon said probe at said given position,and means responsive to the light reflected from said probe at saidgiven position to generate an error cancelling signal whenever saidlight absorbing material is located in said given position. 13.Apparatus for inspecting the condition of a surface having knownirregularities thereon, comprising: means for scanning the surface undertest with a beam of light in a given sequence, said scanning means beingresponsive to variations in the condition of said surface from a givencondition, said scanning means generating erroneous condition variationsignals in response to said known irregularities, preprogrammed meanscoupled to said scanning means programmed to generate error cancellingsignals for causing said scanning means to disregard said erroneouscondition variation signals, an error cancelling signal being generatedfor each of selected ones of said irregularities whereby only saidselected ones of said erroneous signals are cancelled, said scanningmeans including means for causing said beam of light to continuouslyscan said surface, said known irregularities occurring in a given orderduring said sequence, the surface to be inspected including acylindrical bore, said scanning means including an elongated cylindricalprobe rotatably and axially moveable with respect to said bore, and saidpreprogrammed means including a pattern of substantially identicalirregularities secured externally to said probe, arranged insubstantially the same pattern as the known irregularities on saidsurface, each of said identical irregularities corresponding to adifferent one of said known irregularities and being located in a givenposition external the bore when said light beam impinges upon thecorresponding known irregularity.
 14. Apparatus for inspecting thecondition of a surface having known irregularities thereon, comprising:scanning means for optically scanning the surface under test inaccordance with a given program, said scanning means being responsive tovariations in the condition of said surface from a given condition, saidscanning means generating condition variation signals in response toboth known and unknown irregularities, means coupled to said scanningmeans and preprogrammed to follow said given program and to disregardsaid known irregularities whereby only signals for unknownirregularities arE provided as output signals, said surface to beinspected including a cylindrical bore, said scanning means including aprobe rotatably and axially moveable with respect to said bore, saidpreprogrammed means including means simulating said known condition forgenerating in time coincidence with said known irregularities errorcancelling signals for cancelling erroneous condition variation signals,said error cancelling signals causing said scanning means to disregardsaid known irregularities, said preprogrammed means including meanssecured to said probe external to said bore during the scanning of saidbore, said secured means having a given position external said bore whensaid scanning means responds to said known irregularities, and saidpreprogrammed means further including means located at said givenposition responsive to said secured means for generating said errorcancelling signals.
 15. The apparatus of claim 14 wherein said securedmeans includes light absorbing material located on said probe atdiscrete locations, each discrete location corresponding to a differentone of said irregularities, said probe being light reflective, and saidmeans located at said given position includes light generating means forcausing a light beam to impinge upon said probe at said given position,and means responsive to the light reflected from said probe at saidgiven position to generate an error cancelling signal whenever saidlight absorbing material is located in said given position. 16.Apparatus for inspecting the condition of a surface having knownirregularities thereon, comprising: means for scanning the surface undertest with a beam of light in a given sequence, said scanning means beingresponsive to variations in the condition of said surface from a givencondition, said scanning means generating erroneous condition variationsignals in response to said known irregularities, preprogrammed meanscoupled to said scanning means programmed to generate error cancellingsignals for causing said scanning means to disregard said erroneouscondition variation signals, an error cancelling signal being generatedfor each of said irregularities whereby said erroneous signals arecancelled, said scanning means including means for causing said beam oflight to continuously scan said surface, said known irregularitiesoccurring in a given order during said sequence, said surface to beinspected being formed as a cylindrical bore, said scanning meansincluding an elongated cylindrical probe rotatably and axially moveablewith respect to said bore, and said preprogrammed means including apattern of substantially identical irregularities secured externally tosaid probe, arranged in substantially the same pattern as the knownirregularities on said surface, each of said identical irregularitiescorresponding to a different one of said known irregularities and beinglocated in a given position external the bore when said light beamimpinges upon the corresponding known irregularity.